[XCore] Make use of the target independent global address offset folding.

This let us to remove some custom code that matched constant offsets
from globals at instruction selection time as a special addressing mode.
No intended functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181126 91177308-0d34-0410-b5e6-96231b3b80d8